Nanomaterials are materials having a size of 1 nm˜100 nm. They can be synthesized in various shapes such as a sphere, plate, tube and the like. Since nanomaterials have peculiar properties different from conventional bulk materials, applied technologies using such peculiar properties of nanomaterials have been actively developed in various fields of electronics, information, environment, energy, medicine, etc.
Particularly, among nanomaterials, magnetic nanoparticles having magnetic properties have been widely researched for the purpose of biomaterial extraction, magnetic resonance imaging (MRI) contrast agents, biosensors, drug/gene transfer, and high-temperature magnetic medical treatment. Among MRI contrast agents, an MRI liver contrast agent based on iron oxide nanoparticles, Resovist (manufactured by Bayer Schering Corporation), is currently used in clinical practice, and an MRI lymph node contrast agent, Combidex (manufactured by AMAAG Corporation), is known to be under clinical testing.
However, iron oxide nanoparticles, which are magnetic nanoparticles used for commercially-available MRI contrast agents, are synthesized by coprecipitation using a metal salt in an aqueous solution. Therefore, the iron oxide nanoparticles synthesized in this way are disadvantageous in that it is difficult to adjust their size, and in that they have poor monodispersity. Further, since these iron oxide nanoparticles are synthesized at room temperature, there is a disadvantage in that they have low crystallinity.
Further, iron oxide nanoparticles may also be synthesized by thermal decomposition. Thermal decomposition. uses a process of mixing an organic surface stabilizer with an organic solvent and a metal precursor to improve particle stability and then heating the mixture to the boiling point of the organic solvent to age the mixture. Therefore, thermal decomposition is advantageous in that a reaction temperature can be easily controlled, and in that iron oxide nanoparticles of various sizes can be synthesized when solvents having boiling points different from each other are used. The iron oxide nanoparticles synthesized in this way exhibit excellent physical properties in application fields because they have excellent monodisperity and crystallinity compared to iron oxide nanoparticles synthesized in an aqueous solution.
However, thermal decomposition is problematic in that iron oxide nanoparticles are synthesized in an organic solvent, so that the synthesized iron oxide nanoparticles are surrounded by an organic substance, with the result that these iron oxide nanoparticles are not suitable for practical use in nano-bio applications.
Therefore, it is required to surface-treatment the iron oxide nanoparticles using a water-dispersible material that is stable in the body. There are various methods of surface-treatment of nanoparticles. Typically, there are ligand exchange methods of converting the organic substance surrounding nanoparticles into a hydrophilic substance and encapsulation methods of covering nanoparticles with a hydrophilic substance with the nanoparticles surrounded by the organic substance. The encapsulation methods include a method of capsulating one nanoparticle and a method of capsulating several nanoparticles.
In the encapsulation methods, when an amphiphilic compound having both hydrophilicity and hydrophobicity is used, the hydrophobic region of the amphiphilic compound bonds with the surface of nanoparticles, and the hydrophilic region thereof is distributed throughout the outermost shell of the encapsulated nanoparticles, so that water-insoluble nanoparticles can be stably dispersed in an aqueous solution, thereby maximizing the usage rate of nanoparticles in the body.